1. Technical Field
The present invention relates to technology for receiving multi-carrier modulated signals in which a plurality of sub-carriers have been multiplexed.
2. Background Art
Presently, Orthogonal Frequency Division Multiplexing (OFDM) is a broadcasting scheme widely used for various types of digital transmission, notably including terrestrial digital broadcasting and the IEEE 802.11a standards. The OFDM scheme makes highly efficient use of frequencies by frequency multiplexing a plurality of narrowband digitally-modulated signals using mutually-orthogonal sub-carriers.
Additionally, in the OFDM scheme, the length of one symbol comprises the length of a useful symbol as well as the length of a guard interval. As such, a portion of the useful symbol-length signal is reproduced at length the guard interval to produce intra-symbol periodicity. Thus, the influence of inter-symbol interference caused by multi-pass interference can be reduced given that such a scheme offers superb resistance to such interference.
The terrestrial digital television broadcasting scheme employed in Japan, namely ISDB-T (Integrated Services Digital Broadcasting-Terrestrial) uses the broadcasting format shown in FIG. 30. The terrestrial digital television broadcasting scheme employed in Europe, namely DVB-T (Digital Video Broadcasting-Terrestrial) uses the broadcasting format shown in FIG. 31. In FIGS. 30 and 31, the horizontal axis is the carrier (frequency) direction and the vertical axis is the symbol (time) direction.
As shown in FIGS. 30 and 31, in both ISDB-T and DVB-T, pilot signals are scattered and inserted every 12 sub-carriers along the carrier direction and every four symbols along the symbol direction. These pilot signals are called scattered pilot (hereinafter, SP) signals. SP signals are known to transmitters as well as to receivers and are used by receivers for estimation of channel characteristics.
Further, in DVB-T, pilot signals called continual pilot signals (hereinafter, CP) are present in addition to SP signals. CP signals are pilot signals inserted into every symbol of specific sub-carriers and are used for such purposes as CPE (Common Phase Error) elimination. CP signals are also known to transmitters and receivers alike. The positions of the sub-carriers into which CP signals are inserted (hereinafter referred to as CP carriers) are shown for 8 k mode in FIG. 32. It should be noted that the values given in FIG. 32 show the carrier indices of the CP carriers when the carrier index of the useful sub-carrier with the lowest carrier frequency is zero. In ISDB-T, CP signals are inserted into only one sub-carrier.
Carrier frequency synchronisation is necessary to OFDM signal reception. Generally speaking, carrier frequency synchronisation is divided into (i) narrowband carrier frequency synchronisation used for detection and correction of discrepancies within the transmitted sub-carrier spacing (narrowband carrier frequency discrepancies) and (ii) wideband carrier frequency synchronisation used for detection and correction of discrepancies at the unit level of the sub-carrier spacing (wideband carrier frequency discrepancies).
The greater the narrowband carrier frequency discrepancies, the greater the data errors become. Also, the presence of wideband carrier frequency discrepancies causes discrepancies in sub-carrier positions. As such, signal processing is carried out using a different sub-carrier and demodulation cannot be accomplished. This presents difficulties for steady reception.
For these reasons, conventional technology has been proposed for wideband carrier frequency synchronisation. For example, Patent Literature 1 discloses an OFDM signal demodulator that performs wideband carrier frequency synchronisation through correlation of the location of CP signals included in the DVB-T transmission format. The configuration of the OFDM signal demodulator disclosed in Patent Literature 1 is shown in FIG. 33.
In the receiver, the frequency of an OFDM signal input from a channel to the OFDM signal demodulator is converted by the tuner 1001 from the RF (Radio Frequency) band to the IF (Intermediate Frequency) band. The quadrature demodulation circuit 1002 uses a fixed frequency to perform quadrature demodulation on IF band OFDM signals and then outputs baseband OFDM signals so obtained to the fc correction circuit 1003.
The fc correction circuit 1003 generates a corrected carrier frequency according to the narrowband carrier frequency error input from the narrowband fc error calculation circuit 1004 as well the wideband carrier frequency error input from the wideband fc error calculation circuit 1008, and then applies corrections to carrier frequency discrepancies in the baseband OFDM signals according to the corrected carrier frequency so generated.
The baseband OFDM signals in which carrier frequencies have been corrected are supplied to the narrowband fc error calculation circuit 1004 and to the FFT circuit 1005. The narrowband fc error calculation circuit 1004 correlates the guard interval-length signal and the end portion of the useful symbol length-signal within the baseband OFDM signals to calculate the carrier frequency error within the sub-carrier spacing (narrowband carrier frequency error), and then outputs the carrier frequency error so calculated to the fc correction circuit 1003. The FFT circuit 1005 performs a Fast Fourier Transform (FFT) on the useful signal-length portion of the baseband OFDM signals, thus converting same into frequency-domain signals.
The differential detection circuit 1006 calculates inter-symbol phase fluctuations through inter-symbol differential detection on each of the sub-carrier signals in the frequency-domain signals input from the FFT circuit 1005, and then outputs the signals so calculated (hereinafter called differential detection signals) to the correlation circuit 1007 and the phase averaging circuit 1009. The correlation circuit 1007 correlates the differential detection signals from the differential detection circuit 1006 and the location sequence signal of the sub-carriers that transmit the CP signals, then outputs the correlated values to the wideband fc error calculation circuit 1008.
The wideband fc error calculation circuit 1008 detects peak positions in the correlated values input from the correlation circuit 1007, calculates the carrier frequency error at the unit level of the sub-carrier spacing (wideband carrier frequency error) from the peak positions so detected, and then outputs the carrier frequency error so calculated to the fc correction circuit 1003.
The phase averaging circuit 1009 performs intra-symbol averaging of the phases evidenced in the differential detection signals from the differential detection circuit 1006 corresponding to the CP signals, estimates the CPE, then outputs the CPE so estimated to the phase fluctuation correction circuit 1010. The phase fluctuation correction circuit 1010 applies corrections (CPE elimination) to phase fluctuations in the signals output from the FFT circuit 1005 according to the CPE input from the phase averaging circuit 1009, and then outputs signals in which CPE has been eliminated. The detection circuit 1011 detects the signals output from the phase fluctuation correction circuit 1010.
The differential detection circuit 1006 is explained with reference to FIG. 34. In the differential detection circuit 1006, the delay circuit 1031 delays and outputs the signals output from the FFT circuit 1005 by one symbol. The conjugation circuit 1032 calculates and outputs the complex conjugate of the signals output from the delay circuit 1031. The complex multiplier 1033 multiplies the signals output from the FFT circuit 1005 by the signals output from the conjugation circuit 1032, then outputs the complex signals (differential detection signals) so obtained to the correlation circuit 1007 and to the phase averaging circuit 1009.
Next, the correlation circuit 1007 is further explained with reference to FIG. 35. The differential detection signals output from the differential detection circuit 1006 are input to the shift register 1051. The shift register 1051 comprises a plurality of tap outputs corresponding to the positions of the sub-carriers that transmit the CP signals, the output of which is input to the summation circuit 1052. The summation circuit 1052 calculates the sum of the tap outputs of the shift register 1051. The power calculation circuit 1053 calculates the power value of the sum of the tap outputs, then outputs the power value so calculated to the wideband fc error calculation circuit 1008 as the correlated value.
The differential detection signals output from the differential detection circuit 1006 have the same value for every CP carrier position and arbitrary values for non-CP carrier positions. As such, if all the tap outputs of the shift register 1051 are CP carrier positions, then the correlated value output by the correlation circuit 1007 is maximized. The wideband fc error calculation circuit 1008 is able to detect the carrier frequency error at the sub-carrier unit level (wideband carrier frequency error) from the timing at which the correlated value output from the correlation circuit 1007 is so maximized.
Analogue television broadcasts are being phased out in many countries and frequency reconfiguration is being carried out worldwide. In Europe, demand is growing for high definition (HD) service in addition to DVB-T standard definition (SD) broadcasting. Thus the second generation European digital terrestrial television broadcasting system, DVB-T2, has been advanced. As shown in FIG. 36, frames in DVB-T2 include P1 symbols, P2 symbols, and data symbols.
P1 symbols have an FFT size of 1 k and, as shown in FIG. 37, a guard interval before and after the useful symbol length. Unlike the guard intervals previously used in ISDB-T and DVB-T, these guard intervals reproduce the leading half of the useful symbol length before the useful symbol itself, and repeat the ending half of the useful symbol length afterward. Such reproduction is created by shifting a source signal by a predetermined frequency fsh and inserting the signal so obtained into the guard interval portions. Additionally, as shown in FIG. 38, P1 symbols are made up of active carriers and of null (unused) carriers.
P1 symbols include information indicating whether the P2 symbols and the data symbols use MISO (Multiple-Input Single-Output) mode or SISO (Single-Input Single-Output) mode (hereinafter referred to as SISO/MISO information), information indicating the FFT size of the P2 symbols and data symbols (hereinafter referred to as FFT size information), information indicating whether or not FEF (Future Extension Frames) are included (hereinafter referred to as FEF information) and the like.
The P2 symbols and data symbols share a common FFT size and guard interval fraction. The guard interval fraction is the ratio of the guard interval length to the useful symbol length. The combinations of FFT size and guard interval fraction used in DVB-T2 are shown in FIG. 39 together with the permitted pilot patterns for each such combination. There are eight pilot patterns, namely PP1 through PP8. In FIG. 39, “N/A” is used to indicate that no pilot patterns are permitted for a given combination of FFT size and guard interval fraction.
Pilot signals are inserted into the P2 symbols at equal intervals. Such pilot signals are hereinafter referred to as P2 pilot signals. P2 pilot signals are inserted every six sub-carriers when an FFT size of 32 k and SISO mode are used, and are inserted every three sub-carriers otherwise.
P2 symbols include information indicating the pilot pattern for the data symbols (hereinafter referred to as pilot pattern information), information indicating whether the carrier mode is extended or normal (hereinafter referred to as carrier mode information), the number of symbols per frame, the modulation scheme, the encoding ratio of forward error correction (FEC) codes, and other such information and transmission parameters necessary for reception. The number of P2 symbols per frame depends on the FFT size as shown in FIG. 40.
In DVB-T2, extended mode is defined so as to have an extended number of useful sub-carriers. FIG. 41 shows the sub-carrier positions in the two carrier modes, namely normal mode and extended mode. Normal mode uses, as useful sub-carriers, a first range of sub-carriers which consists of the middle portion of all sub-carriers to the exclusion of a plurality of sub-carriers with the highest frequencies as well as a plurality of sub-carriers with the lowest frequencies. Extended mode uses, as useful sub-carriers, a second range of sub-carriers which includes the first range as well as a predetermined number of sub-carriers with the highest and lowest frequencies. That is, extended mode extends beyond normal mode at the left and right ends thereof. FFT sizes of 8 k, 16 k, and 32 k can be selected in extended mode, and P2 symbols and data symbols are applicable thereto.
The number of useful sub-carriers in the two modes for each FFT size is shown in FIG. 42. The number of useful sub-carriers is greater in extended mode than in normal mode and as such, more information can be transmitted through the use thereof. In FIG. 42, “N/A” is used to indicate that a given FFT size cannot be used in extended mode. Also, given that extended mode cannot be used with FFT sizes of 1 k, 2 k, and 4 k, the value of Δf, which indicates half the difference in the number of useful sub-carriers, cannot be calculated and is replaced with a dash (-).
Much like in DVB-T and in ISDB-T, SP signals are inserted into the data symbols, as are CP signals for predetermined sub-carriers. However, in DVB-T2, the location pattern of these SP signals and CP signals is determined according to the pilot pattern in use.
(Math. 1) gives the SP signal location for each of the pilot patterns PP1 through PP8 in normal mode.k mod(DxDy)=Dx(l mod Dy)  (Math. 1)
(Math. 2) gives the SP signal location for each of the pilot patterns PP1 through PP8 in extended mode.(k−Kext)mod(DxDy)=Dx(l mod Dy)  (Math. 2)
In both (Math. 1) and (Math. 2), mod is the modulo operator, k is the useful sub-carrier number, and l is the symbol number. Kext is the value of half the difference in the number of useful sub-carriers between normal mode and extended mode (Δf in FIG. 42). Furthermore, as shown in FIG. 43, Dx is the sub-carrier interval between the positions of any two sub-carriers having SP signals and Dy is the symbol interval between SP signals within a single sub-carrier. In FIG. 43, the horizontal axis is the carrier (frequency) direction and the vertical axis is the symbol (time) direction.
The values of Dx and Dy for each pilot pattern PP1 through PP8 are given in FIG. 44.
Within a given symbol, the sub-carrier interval between the sub-carrier positions of SP signals is DxDy, as given in FIG. 44. The SP signal location pattern previously shown for DVB-T and ISDB-T corresponds to PP1 as given by FIG. 44.
FIG. 45 and FIGS. 46 through 49 show the CP signal location patterns corresponding to the pilot patterns PP1 through PP8. FIG. 45 shows the groups CP_g1 through CP_g6 used by FFT size. If two or more groups (CP_g1 through CP_g6) are indicated, then all such groups are used at once. FIGS. 46 through 49 show the values that belong to the groups CP_g1 through CP_g6 and that correspond to the pilot patterns PP1 through PP8.
Let K be the value indicated in FIGS. 46 through 49 and let N be the value indicated in FIG. 45. The value of KmodN is then a useful sub-carrier number of a CP signal. Here, mod is the modulo operator. It should be noted that for an FFT size of 32 k, the useful sub-carrier numbers of CP signals are the values indicated in FIGS. 46 through 49 with no modulo operation performed therewith. In FIG. 45, N values corresponding to the FFT size of 32 k are given as a dash for this reason.
In normal mode, the values obtainable from FIG. 45 and FIGS. 46 through 49 are the useful sub-carrier numbers. In extended mode, the values indicated in FIG. 50 are additional useful sub-carrier numbers of CP signals that supplement the useful sub-carrier numbers obtainable from FIG. 45 and FIGS. 46 through 49. No modulo operation is necessary for the values in FIG. 50. In FIG. 50, “N/A” is used to indicate that a given combination of FFT size and guard interval fraction is not permitted. Furthermore, “None” is used to indicate the absence of additional sub-carriers with CP signals.
In normal mode, the useful sub-carrier numbers for SP signals and CP signals are given as follows: the useful sub-carrier with the lowest frequency is used for reference and the number thereof is set to zero. The useful sub-carrier numbers are set so as to increase along with increasing frequency. In extended mode, the effective carrier numbers for SP signals and CP signals are given as follows: the useful sub-carrier with the lowest frequency is used for reference and the number thereof set to zero. The useful sub-carrier numbers are set so as to increase along with increasing frequency.
As shown in Non-Patent Literature 1, there exists a method for realising wideband carrier frequency synchronisation using P1 symbols in receiver technology for the DVB-T2 transmission format as described above. The configuration of such a receiver is shown in FIG. 51.
In the receiver, the frequency of OFDM signals input thereto from channels is converted by the tuner 2001 from the RF band to the IF band. The quadrature demodulator 2002 uses a fixed frequency to perform quadrature demodulation on the IF band OFDM signals and then outputs the baseband OFDM signals so obtained to the fc corrector 2003.
The fc corrector 2003 generates a corrected carrier frequency according to the narrowband carrier frequency error input from the narrowband fc error calculator 2005 as well the wideband carrier frequency error and the narrowband carrier frequency error input from the P1 demodulator 2004, and then applies corrections to carrier frequency discrepancies in the baseband OFDM signals according to the corrected carrier frequency so generated.
The baseband OFDM signals in which carrier frequency discrepancies have been corrected are supplied to the P1 demodulator 2004, to the narrowband fc error calculator 2005, and to the FFT unit 2006.
The P1 demodulator 2004 detects P1 symbols that are included in the DVB-T2 transmission format in the baseband OFDM signals input from the fc corrector 2003. The P1 demodulator 2004 detects the wideband carrier frequency error and the narrowband carrier frequency error for the P1 symbols and applies corrections to carrier frequency discrepancies therein, then outputs the wideband carrier frequency error and the narrowband carrier frequency error so detected to the fc corrector 2003. Additionally, the P1 demodulator 2004 decodes the P1 symbols and outputs the control information obtained as a result thereof to the control information collector 2010.
The narrowband fc error calculator 2005 uses correlation (guard correlation) between the guard interval-length signal and the end portion of the useful symbol length-signal of the P2 symbols or data symbols to calculate the carrier frequency error within the sub-carrier spacing (narrowband carrier frequency error) for each such symbol, and then outputs the narrowband carrier frequency error so calculated to the fc corrector 2003.
The FFT unit 2006 performs Fast Fourier Transforms on the time-domain baseband OFDM signals input from the fc corrector 2003 then outputs the resulting frequency-domain baseband OFDM signals to the channel characteristics estimator 2007 and to the equaliser 2008. The channel characteristics estimator 2007 estimates the channel characteristics, i.e. the amplitude and phase displacement, of the channel through which the frequency-domain baseband OFDM signals input from the FFT unit 2006 were received, then outputs the channel characteristics so estimated to the equaliser 2008. The equaliser 2008 uses the channel characteristics estimated by the channel characteristics estimator 2007 to correct the amplitude and phase displacement of the frequency-domain baseband OFDM signals input from the FFT unit 2006, then outputs the signals so corrected to the error corrector 2009.
The error corrector 2009 corrects errors in the signals input from the equaliser 2009 and outputs transmission parameters and other such control information transmitted in the P2 symbols to the control information collector 2010.
The control information collector 2010 classifies the control information collected from the P1 demodulator 2004 and from the error corrector 2009 into transmission parameters.
The P1 demodulator 2004 is explained with reference to FIG. 52. The baseband OFDM signals output from the fc corrector 2003 are input to the P1 position detector 2101 within the P1 demodulator 2004.
The P1 position detector 2101 calculates the correlation (guard correlation) between the guard interval-length signal and predetermined portions of the useful symbol-length signal of the P1 symbols for the baseband OFDM signals input from the fc corrector 2003 and detects the positions of P1 symbols from the peak value of integrals taken over the guard interval. This correlation calculation is carried out with the frequency shift fsh added by the transmitter taken into consideration. Furthermore, the predetermined portions are the leading portion of the useful symbol for the guard interval that precedes the useful symbol and the closing portion of the useful symbol for the guard interval that follows the useful symbol.
The P1 narrowband fc error detector and corrector 2102 (hereinafter referred to as P1 NAFC) detects the carrier frequency error within the P1 symbol sub-carrier spacing (narrowband carrier frequency error) from the guard correlation between the guard interval-length signals of P1 symbols and predetermined portions of the useful symbol-length signals based on the P1 symbol positions detected by the P1 position detector 2101, and also applies corrections to narrowband carrier frequency discrepancies for the P1 symbols according to the narrowband carrier frequency error so detected. The P1 NAFC 2102 outputs the P1 symbol narrowband carrier frequency error so detected to the fc corrector 2003 and outputs the P1 symbols in which narrowband carrier frequencies have been corrected to the FFT unit 2103.
The FFT unit 2103 performs FFTs on the P1 symbol time-domain baseband OFDM signals input from the P1 NAFC 2102 and outputs a P1 symbol frequency-domain baseband OFDM signal to the P1 wideband fc error detector and corrector 2104 (hereinafter referred to as P1 WAFC).
The P1 WAFC 2104 detects the carrier frequency error at the unit level of the P1 symbol carrier spacing (wideband carrier frequency error) and also applies corrections to wideband carrier frequency discrepancies for the P1 symbols according to the wideband carrier frequency error so detected. The P1 WAFC 2104 outputs the P1 symbol wideband carrier frequency error so detected to the fc corrector 2003 and outputs the P1 symbols in which wideband carrier frequencies have been corrected to the P1 decoder 2105.
The P1 decoder 2105 decodes the P1 symbols input from the P1 WAFC 2104 and extracts FFT size information, MISO/SISO information and the like therefrom.
The detection of the P1 symbol wideband carrier frequency error is explained below.
As described above, P1 symbols contain active carriers and null carriers. These are used to calculate the power of each sub-carrier signal as well as to correlate the results of such calculations with a known location sequence of active carriers. Given that active carriers are modulated using BPSK, correlation with a shift when the wideband carrier frequency error is zero gives the sum of all active carriers and thus results in a larger value in comparison to correlated values using other shifts which incorporate null carriers. Therefore, the shift obtained from the largest correlated value is the wideband carrier frequency error, which in turn makes detection thereof possible.